Oxygen-17 excesses of the Central Namib gypcretes: spatial distribution

Earth and Planetary Science Letters 192 (2001) 125^135 www.elsevier.com/locate/epsl

Oxygen-17 excesses of the Central Namib gypcretes: spatial distribution Huiming Bao a; *, Mark H. Thiemens a , Klaus Heine b a

Department of Chemistry and Biochemistry, University of California San Diego, Mail Code 0356, La Jolla, CA 92093, USA b Institute of Geography, University of Regensburg, 93053 Regensburg, Germany Received 27 November 2000; accepted 13 July 2001

Abstract We present here sulfate oxygen isotopic data (72 samples with both N 18 O and N 17 O) systematically collected from the Central Namib Desert. Surface soils from two shore-inland (west^east) transects exhibit a gradual increase in the sulfate oxygen-17 excess (v17 O = N 17 O30.52 N 18 O) until at ca. 70 km inland, where no continuous gypcrete deposit is observed further east (inland). The oxygen isotopic compositions for water-soluble sulfates extracted from soils and gypcretes range from 8.3 to 13.3x and 0.06 to 1.11x for N 18 O and v17 O, respectively. The lateral pattern is similar to what has been seen in the cold deserts of the Antarctic dry valleys. However, unlike the dry valleys, no discernible correlation is found between N 18 O and v17 O, or between the depth of soil horizon and v17 O in the Namib. Possible explanations include a relatively smaller component of dimethylsulfide (DMS)-derived sulfate in the total gypsum deposits and/or more active surface processes (e.g., flooding and leaching) in the Central Namib Desert than in the Antarctic cold deserts. Although current state of knowledge is insufficient to delineate quantitatively the sulfate contributions from different sources and reactions, the measurement of sulfate v17 O does identify an unmistakable atmospheric sulfate component and provides additional independent information regarding sources and reactions. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: Namib Desert; Desert soils; gypsum; sulfates; O-18/O-16; O-17/O-16; atmospheric precipitation; dimethyl sul¢de

1. Introduction As a measure of the deviation from mass-dependent isotopic compositions, oxygen-17 excess, v17 O, is quanti¢ed by the value of N 17 O30.52U

* Corresponding author. Present address: Dept. Geology and Geophysics, Louisiana, State University, E235 HoweRussell Geoscience Complex, Baton Rouge, LA 70803, USA. Fax: +1 225-578-2302. E-mail address: [email protected] (H. Bao).

N 18 O. Since the discovery of oxygen-17 excess in terrestrial rocks [1], positive sulfate v17 O values have been found to be almost ubiquitous in desert environments [2], the Antarctic dry valleys [3], building surfaces (Bao, unpublished data), and some ancient volcanic ash-beds [1]. Recent sulfate oxygen isotopic data from rainwater and aerosols, as well as from controlled laboratory experiments suggested that the observed oxygen-17 anomalies in sulfate minerals ultimately derive from atmospheric sulfur oxidation by ozone (O3 ) and hydrogen peroxide (H2 O2 ), both of which possess

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 4 4 6 - 0

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positive v17 O [4,5]. Due to the non-labile characteristics of oxygen in sulfate ions, exchange with ambient water or other oxygen-bearing compounds is exceedingly slow under most surface conditions [6,7]. Thus, the oxygen isotopic signatures of atmospheric O3 and H2 O2 are inherited and preserved in sulfate. Such sulfate is physically

removed from the atmosphere by wet and dry depositions and may accumulate as a signi¢cant geological record in certain surface environments, and provide clues to ancient environmental and climatic conditions, location of large upwelling current, and atmospheric chemical processes. Gypcrete soils (gypsic Aridisols) in the Central

Fig. 1. Map showing the geographic location of in the Central Namib Desert.

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Fig. 2. (A) View over the Namib Desert surface west of Gobabeb. Desert pavement in the foreground. Car track for scale. Sand dunes in the background. The gypcrete is found beneath the desert pavement (photo: K. Heine). (B) Fully developed gypcrete of the Namib plain near Gobabeb. The uppermost layer documents scouring processes. Gypcrete formation starts about 15 cm beneath the surface. The lower part of the pro¢le ( s 40 cm depth) shows a massive calcrete (photo: R. Walter).

Namib Desert occur extensively near the coast and gradually thin o¡ and disappear at V50^70 km from the coast, suggesting an origin closely related to the ocean in the west (Fig. 1). Gypsum has all conceivable morphologies and occurs mostly in the upper soil horizons (Fig. 2). Gypcrete soils cover an area of V30 000 km2 , mostly in the dune-free part of the Namib Desert [8,9]. Past studies of the sulfate deposit generated several di¡erent views on its origin, including decomposition of marine biogenic H2 S from the organicrich sediments [10] and a bedrock source [11]. Recent N 34 S data, combined with geological, hydrological, and meteorological information, however, suggest an atmospheric origin through the oxidation of reduced sulfur compounds, mostly dimethylsul¢de (DMS) of biological origin from the nearby ocean [9], a conclusion supported by our ¢nding of the oxygen-17 excesses in the gypcretes [1]. In this study, we conducted a survey of sulfate v17 O values in the surface soils and gypcretes from two areas in the Central Namib, the Messum River and the Kuiseb^Tumas River areas (Figs. 1 and 3). In addition to our previous work on three vertical soil pro¢les, we analyzed ¢ve more vertical soil pro¢les in the Kuiseb^Tumas

River area (Fig. 2). The aim of this study is to provide detailed spatial distributions (lateral and vertical) of sulfate oxygen isotopic compositions in the Central Namib Desert and to compare the pattern with that of the cold desert (dry valleys) in Antarctica. 2. Sample locations and analytical method Surface loose soil samples including those below desert pavement were systematically sampled from the coast to 180 km inland (Figs. 1^3). The Central Namib Desert consists of vast rock-cut and gravel-veneered plains that rise with ca. 1³ inclination over 100^150 km from the sea level to about 1000 m above sea level at the base of the Great Escarpment. A series of ephemeral river courses drains the area. These rivers have catchments ranging in size from less than 2000 km2 to more than 30 000 km2 . The climate of the area is hyper-arid. Rainfall at the coast averages about 10^20 mm/a and increases eastwards towards the Great Escarpment where it may exceed 200 mm/a. Heavy showers may occur and cause selective removal of ¢ne soil material on the desert surface.

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Fig. 3. Enlarged map showing surface sample localities of the Messum River and the Kuiseb^Tumas River areas, Central Namib Desert.

The coast of the Central Namib Desert receives up to 34 mm/a of moisture only by fog precipitation on 65 days in the year and isolated inselbergs (e.g., Swartbank, 50 km inland) up to 180 mm/a on 85 days/a [12,13]. As a result of moist oceanic air £owing over the upwelled cold water of the Benguela Current, the e¡ects of fog are felt more than 100 km inland. Temperatures are oceanic at the coast with low seasonal and daily ranges (5³C and 8³C respectively). There is a steep rise in maximum temperatures between the coast and the stations further inland. In the Namib Desert conditions of very low relative humidity are rare and are associated with berg winds from the East during winter. The Central Namib may experience high velocity berg winds (up to 60 km/h) [14]. Wind streaks on rock outcrops in the Central Namib Desert document the erosive energy of the berg winds [15]. Information about soil moisture conditions from di¡erent places of the Central Namib Desert are not available, yet the gypcretes document that now and then soil

water penetrates from the surface into the solum [8]. The two sampled areas di¡er in geology and relief. In the northern, partly mountainous area of the Messum River, Palaeozoic granites, pegmatites, sandstones and shales as well as Mesozoic £ood basalts (Etendeka) crop out. In the south, the Kuiseb^Tumas area, Precambrian/Palaeozoic granites, gneisses, shists, quartzites and marbles build the plains and scattered inselbergs. In both areas samples were taken from sites situated on the old erosion surfaces. In the valleys and on slopes soils consist of either alluvial and colluvial deposits or are mantled by rock debris, respectively, and do not show gypcretes. Gypsic Aridisols normally developed on older erosion surfaces of the Namib Desert [8], which is consistent with an atmospheric origin of gypsum that requires stable and long-term accumulation. In many places a desert pavement ^ coarse-grained armour deposits found on the surface of many deserts ^ formed on top of the soil pro¢les. A total of eight vertical soil pro¢les were sampled from the Kuiseb^Tumas area. Pro¢le pits about 1 m wide were dug down to the bedrock or to massive carbonate cementation (Fig. 2). The surface samples (Figs. 2 and 3, and Table 1) were taken from to uppermost centimeters of the loose soil or gypcrete material disregarding the coarser particles. In places where a desert pavement amours the loose soil material, the samples were taken immediately beneath the stony surface layer. The samples were stored in plastic bags and dried (45³C) in the laboratory. Approximately 10^40 g loose soil samples were soaked in Millipore water (doubly deionized). After shaking, centrifugation, ¢ltration, evaporation, and acidi¢cation, water-soluble sulfate was precipitated as barite (BaSO4 ) by adding saturated BaCl2 solution. For soil samples containing gypsum crystals, much less quantity is needed. Oxygen isotopic analysis on barite was done by a CO2 -laser £uorination system with an analytical precision of þ 0.8x and þ 0.05x for N 18 O and v17 O, respectively [16]. The co-variations between N 18 O and N 17 O along the entire analytical procedure resulted in highly reproducible v17 O values. All isotopic data are reported in standard N notation with respect to SMOW.

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3. Results We found that very little water-soluble sulfate may be extracted from surface loose soils at distances s 55 km east of the coast. Therefore no isotopic data are available except for one rock varnish sample at V82 km, an isolated gypcrete

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at 73.4 km, and one soil sample in Solitaire (150 km east of the coast) (Table 1, Figs. 3 and 4). All N 18 O values of these surface samples are very close, ranging from 8.3x to 13.3x, with an average value 10.9 þ 1.2x (n = 71), slightly more positive than the seawater sulfate value 9.4x. The v17 O value for sulfates from surface soils

Table 1 Sulfate oxygen isotopic compositions and O-17 excesses from surface materials in the Central Namib Desert Sample Messum River area 099-38

N18 O

N17 O (SMOW)

v17 O

Distance to the nearest shore (km)

8.3

4.4

0.06

0.1

099-33 099-34 099-37 099-35 099-36 099-32 099-31 099-30 099-29

10.1 10.3 12.2 9.4 11.1 10.5 11.0 9.7 9.7

5.5 5.5 6.5 4.9 5.8 5.8 6.1 5.5 6.1

0.23 0.18 0.21 0.03 0.02 0.32 0.39 0.51 1.11

0.5 0.5 1.0 1.3 1.3 5.7 10.0 21.3 34.0

099-28 099-22

8.4 13.3

4.8 7.3

0.44 0.44

42.6 74.5

Kuiseb^Tumas River areas 099-56 11.9 099-60 10.7

6.4 5.9

0.19 0.36

6.5 8.7

099-57

9.9

5.5

0.31

9.0

099-58 099-59 099-55 099-54 099-62 099-63

12.5 11.6 11.8 9.6 11.1 10.1

6.8 6.4 6.4 5.4 6.1 5.7

0.27 0.33 0.26 0.41 0.34 0.50

9.0 11.2 13.7 18.5 29.4 38.8

099-69 099-70

12.1 9.9

6.7 5.6

0.42 0.44

40.4 44.7

099-67 099-64

12.0 10.3

6.7 5.9

0.42 0.51

45.3 47.5

099-66

8.9

5.6

0.99

54.9

099-65 099-77

9.8 10.8

5.9 5.9

0.80 0.33

55.0 73.4

9.8

5.0

30.07

151.1

Solitaire area 099-87

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Description

surface soil below desert pavement surface gypcrete vanish; site as 099-33 surface soil surface soil salt; site as 099-35 surface soil surface soil surface soil gypcrete blow desert pavement surface soil varnish on desert pavement Surface sandy soil surface soil below desert pavement surface soil below desert pavement gypcrete; site as 099-57 surface gypcrete surface soil surface soil gypcrete surface soil below desert pavement surface soil surface soil below desert pavement surface gypcrete surface soil below desert pavement surface soil below desert pavement surface gypcrete gypcrete below desert pavement soil 0.4 m below the surface

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Fig. 4. Variations of sulfate v17 O with distance to the ocean. Note that the gypcrete deposit disappears at a distance of 50^60 km to the ocean.

and gypcretes ranges from 0.06 to 1.11x, with an average value 0.39 þ 0.21x. We see a pronounced positive correlation between v17 O values and the distance to the nearest coast, with the lowest v17 O values near the shore. No systematic variation in v17 O value is observed for samples from di¡erent horizons among eight analyzed soil pro¢les, although a few of them have higher v17 O values in deeper horizons (Table 2). There is also no evident correlation between v17 O and N 18 O values (Fig. 5). 4. Discussion There are potentially multiple sources of sulfate for the Namib gypcrete. Sea salt sulfate can be predominant in the close vicinity of the shore. Sulfate of atmospheric origin is certainly an important component as indicated by sulfate's positive v17 O values. Sulfate from bedrock or weathering processes is also a possible minor source. While conventional N 18 O and N 34 S isotopic measurements are helpful in deciphering the contribution of these di¡erent sources, the conclusion is often ambiguous due to overlapping isotopic compositions among varying sources. In this study we show that sulfate v17 O provides additional independent information to this problem. The oxidation pathway of reduced sulfur gases in the atmosphere is not unique. H2 O2 , O3 , OH radical, and air O2 (when catalyzed by metal ions) can all act as oxidants under suitable conditions. At

present, the knowledge of isotopic fractionation in many individual reaction steps of these di¡erent oxidants as well as their reaction kinetics is limited. The oxidation of reduced sulfur compounds in the atmosphere may go through aqueous-phase or gas-phase reactions. A recent comprehensive examination of tropospheric SO2 oxidation pathways using a gas^aqueous photochemical model [17] suggests that oxidation is mostly in gas-phase at low liquid water content (H2 O = 3U1034 g/m3 ) but is dominated by H2 O2 (aq) and O3 (aq) when the pH is below 5 and v6, respectively. At pH between 5 and 6, OH (aq) becomes an important oxidant for SO2 when H2 O2 (aq) is depleted. Sulfur isotopes: In theory, sulfate formed from gas-phase reaction should have lower N 34 S value than that of the source SO2 , while a higher sulfate N 34 S value than that of source SO2 may be obtained from aqueous-phase oxidation [18^20]. In nature, both processes may be in operation. Overall, observations show that the oxidations of SO2 in the atmosphere result in a small increase in N 34 S value (0^4x) for the product sulfate [18,21^23]. The N 34 S value of marine biogenic sulfur (mostly DMS) is not well established, with a theoretical range of 14^22x (CDT) [24] and a few measurements with in that range [25] and the references therein. Thus, sulfate derived from DMS may have N 34 S values ranging from 13^ 26x. Since seawater sulfate has a uniform N 34 S value 21x [26], it is not possible to accurately estimate the relative contribution of sea salt sulfate and DMS-derived sulfate in Namib gypcrete using N 34 S. The Central Namib gypcrete has N 34 S values ranging from 13.0 to 18.8x [9], which is consistent with an exclusive DMS source, but is also consistent with mixture sources of sea salt sulfate, DMS sulfate, and bedrock weathering. Oxygen isotopes: The situation for oxygen isotopes is more complex than for sulfur isotopes due to the involvement of numerous oxygen-bearing species and their di¡erent pathways and kinetics. It is well established that the oxygen isotopes of SO2 exchange readily with ambient moisture, while product sulfate records part of the oxygen signature derived from oxidants [7,27]. Upon oxidation in the atmosphere, the

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product sulfate generally has a higher N 18 O value than that expected from aqueous SO23 3 oxidation by air O2 [28]. These observations seem to be consistent with the involvement of H2 O2 or O3

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as oxidants, since both H2 O2 and O3 have N 18 O values higher than that of air O2 . In the troposphere, H2 O2 and O3 have N 18 O values ranging from 21.9 to 52.5x [29], and 95 to 125x [30],

Table 2 Sulfate oxygen isotopic data from eight di¡erent vertical soil pro¢les, Kuiseb^Tumas area, the Central Namib Desert Sample

N18 O

N17 O

v17 O (SMOW)

Depth below the surface (cm)

Distance from the shore (km)

SWA3-3 SWA3-2 SWA3-4 SWA3-5

11.8 12.8 10.3 10.2

6.6 7.0 5.9 5.8

0.44 0.37 0.56 0.55

14 (wedge-¢ll) 20 26 48

35

SWA7-1 SWA7-2 SWA7-3 SWA7-5 SWA7-6 SWA7-7

11.8 13.1 10.6 11.6 10.0 11.4

6.5 7.0 5.8 6.5 5.5 6.7

0.33 0.20 0.28 0.44 0.35 0.72

1 5 10 18 48 65

41

*SWA6-1 *SWA6-2 *SWA6-3 *SWA6-4 *SWA6-6 *SWA6GYP *SWA6-7

12.6 12.0 11.4 13.2 13.0 11.1 10.1

7.0 6.6 6.3 7.3 7.1 6.2 5.8

0.40 0.31 0.34 0.40 0.37 0.38 0.51

1 9 (crack) 5 24 50 52 70

42

SWA5-2 SWA5-3

10.0 13.0

5.2 7.1

0.01 0.37

1 9

49

*GBB17-5 *GBB17-6 *GBB17-8

9.8 9.5 11.3

5.3 5.2 6.1

0.20 0.26 0.26

16 46 66

61

AUS3-3 AUS3-5 AUS3-6 AUS3-7 AUS3-8

10.3 11.5 8.9 9.6 11.0

5.5 6.2 5.0 5.3 6.0

0.16 0.26 0.29 0.27 0.24

9 30 50 70 90

72

*GOR13-2 *GOR13-3 *GOR13-5 *GOR13-6

12.4 8.3 10.5 11.6

6.8 4.6 5.9 6.4

0.34 0.33 0.42 0.35

5 16 36 52

83

GOR9-1 GOR9-2 GOR9-3 GOR9-4 GOR9-5 GOR9-6 GOR9-7

9.4 10.0 11.8 12.1 10.8 10.1 9.4

5.5 6.0 7.1 6.8 6.1 5.8 5.3

0.56 0.78 0.91 0.51 0.45 0.54 0.43

1 6 10 20 25 32 50

88

*These three soil pro¢les are from [1].

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Fig. 5. Correlations between N17 O and N18 O (a) and between v17 O and N18 O values (b). Error bars are not shown in (a).

respectively. Holt and Kumar [31] derived a comprehensive relationship N 18 O (sulfate) = 3/5 N 18 O(H2 O)+2/5 N 18 O (H2 O2 )+8.4x through their experiments of aqueous oxidation of SO2 by H2 O2 , although more recent work on the same experiment using v17 O suggests half of the sulfate oxygen are derived from H2 O2 [5]. However, H2 O2 is not the only oxidant responsible for sulfur oxidation in the atmosphere, O3 , OH radical, and air O2 may also play important roles under certain conditions. For example, if O3 is involved in the oxidation processes, the product sulfate may incorporate 1/4 of the O3 oxygen that often has a much greater N 18 O value [5]. Meanwhile, kinetic isotopic fractionations also have a signi¢cant e¡ect on the N 18 O value of product

sulfate during oxidation by H2 O2 , whose N 18 O value may increase with decreasing concentration [29]. The gas or aqueous SO2 +OH reaction, perhaps signi¢cant in normal rainwater pH ranges, is one of those other variables that has not been well constrained. As a result, atmospheric sulfate N 18 O values have a large variability, rendering using N 18 O alone for source diagnostics impractical. Mass-independent oxygen isotopic composition: so far, the only processes that could generate positive sulfate v17 O are atmospheric sulfur oxidations by O3 and H2 O2 [5]. In the troposphere, O3 has v17 O values ranging from 20 to 35x [30]. Stratospheric O3 may have higher N 18 O and v17 O values than the tropospheric one but with large uncertainties [32]. Limited measurement on H2 O2 in rainwater yielded a v17 O value ranging from 1.2 to 2.4x [29]. These oxidants pass along their unique v17 O values with characteristic proportions onto product sulfate and are preserved. Air O2 oxidation of reduced sulfur compounds catalyzed by iron and manganese in the atmosphere is a possible pathway, as is oxidation by OH radical in cloud water [33]. However, experiments show that air O2 or OH radical do not produce v17 O-positive sulfate [5]. As mentioned above, the pathway of aqueous sulfur (IV) oxidation depends strongly upon pH, with H2 O2 as the dominant oxidant at pH 6 4^5 but O3 at pH s 6 and OH in between when H2 O2 is depleted [17,33]. Fog water collected from the Central Namib Desert shows pH values between 5.6 and 6.6, with most larger than 6.0 [34]. The pH value of cloud or fog may even be higher in the marine boundary layer since the formation of sulfate reduces the pH. Thus, O3 could be the predominant oxidant in S (IV) oxidation in Central Namib. If 1/4 of the oxygen ended up in product sulfate is from O3 , the sulfate should have a v17 O of ca. 5^8x. The small v17 O values (ca. 0.4x) of gypcrete in Namib suggest that possibly less than 10% of the sulfate deposited in the desert is from O3 oxidation of biogenic sulfur gases. The rest of the sulfates are from sea salt, bedrock weathering, and perhaps from oxidation by air O2 or OH radical that produces sulfate of zero v17 O. Another source of sulfate in the atmosphere is the so-called primary sulfate

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resulted from high-temperature oxidation of SO2 to SO3 accompanied by hydration to H2 SO4 during many human industrial activities [35]. Being signi¢cant in current atmospheric sulfate budget, this anthropogenic source is, nevertheless, negligible for the Namib gypcretes that have been forming for several millions of years. So far, there is insu¤cient knowledge to delineate quantitatively these di¡erent origins. We also cannot exclude the role H2 O2 may play in the oxidation processes due to the lack of measurements of the cloud and fog chemistry in the Central Namib Desert. However, the positive v17 O points unambiguously to an atmospheric source of sulfate that is derived from oxidation of reduced sulfur gases. Near the coast of Central Namib, gaseous sulfur is most likely dominant by the DMS released by biological activities that are enhanced by the Benguela Current [9,36]. This upwelling current exists since ca. 10 000 000 years ago [37] and is a viable source of the extensive gypsum deposits in the Central Namib Desert. The gradual increase of v17 O further inland (Fig. 4) can be interpreted as a reduced sea salt sulfate component due to limited transport distance of micron-sized sea salt particles [33], a pattern very similar to that of the dry valleys in the Antarctica [3]. The highest v17 O value is found at ca. 50^70 km inland where gypcrete is no longer seen further east. Sulfates extracted from soil samples s 70 km inland were not su¤cient for isotopic analysis. A few of the inland data points indicate soil sulfate's v17 O decreases eastward and approaches zero. This change is expected because most rainwater precipitation in Central Namibia comes from the northeast and climate becomes moister in the east. A wet soil condition facilitates microbiological activities and sulfur recycling, which may erase sulfate v17 O signature derived from the atmosphere. We do not see an evident v17 O and N 18 O correlation along the west^east (shore-inland) transects (Fig. 5) as we saw in the dry valleys. Assuming the sulfur oxidation pathway is similar in the two di¡erent regions, we can reasonably predict that the fraction (e.g., 1/4 or 2/5) of the oxygen in product sulfate that originated from oxidants or speci¢c oxidation pathway have a similar oxygen isotopic com-

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position. Since the sulfate oxygen isotopic composition will be largely a¡ected by the rest of the oxygen (3/4 or 3/5) that may have equilibrated with local moisture that have zero v17 O values, the di¡erence in total soil sulfate v17 O value re£ects the di¡erent mixture of sea salt sulfate and atmospheric derived sulfate. Similarly, the sulfate N 18 O value in this case re£ects mainly the N 18 O value of local moisture, and the total soil sulfate N 18 O value represents the combination of sea salt sulfate value (9.4x) and that of the atmospheric sulfate that are expected to be very negative in Antarctica. Thus, because the large di¡erences in N 18 O and v17 O values between sea salt sulfate and the pure biogenic sulfate (derived from atmospheric oxidation), we see an obvious trend of increasing soil sulfate v17 O with increasing distance from the sea in the Antarctic dry valleys. A smaller N 18 O di¡erence between sea salt sulfate and sulfate derived from DMS oxidation, or a small percentage of DMS-derived sulfate in the total may explain the lack of trend in the Central Namib. Similarly, a distinct pattern of increasing sulfate v17 O down the soil pro¢les observed in the dry valleys is not obvious here in the Central Namib Desert (Table 2). While some soil pro¢les have the highest v17 O values in deep horizons, the overall trend is rather variable. The existence of periodic £ooding events [38], a relatively intensive soil leaching [39,40] compared to that in the dry valleys, and perhaps a small percentage of DMS-derived sulfates in the total in this region might all contribute to the observed v17 O vertical pro¢les. At present, the inadequate understanding of the processes and magnitude of isotopic fractionation for each of the possible reactions involved in S (IV) oxidation in the atmosphere prevents us from acquiring deeper insight into many important environmental parameters that may be recorded in the atmospheric sulfate isotopic signatures. One potential use of atmospheric sulfate is as a tracer of the N 18 O of atmospheric moisture. Soil sulfate N 18 O values range from 311 to 3x in the Antarctic dry valleys, while from 8.3x to 13.3x in the Namib. Excluding those caused by di¡erent sea salt components, this sulfate N 18 O di¡erence may be due in large part to the di¡er-

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ence in the N 18 O of local moisture. Upon resolution of the factors a¡ecting oxygen isotopic compositions of atmospheric sulfate, it will ultimately be possible to delineate the oxygen contributions of various sources and processes. 5. Conclusion

[2] [3]

[4]

A lateral v17 O pattern for sulfate in the Central Namib Desert is found to be similar to that in Antarctic dry valleys. Both exhibit a stronger in£uence of sea salt sulfate in the coastal zone than further inland. Both N 18 O and v17 O have narrower ranges in the Namib than in the Antarctica, contributing possibly in part to the lack of correlation between N 18 O and v17 O, and between v17 O and soil depth in the Central Namib. A more active surface modi¢cation process in the Namib relative to that in the dry valleys may also be responsible for the poor or no correlation. The di¡erences in the ranges of sulfate N 18 O in the Namib and in the Antarctica indicate apparently that the N 18 O signature of local moisture as well as local temperature is recorded in atmospheric sulfate oxygen isotopic compositions. The ultimate delineation of the information awaits fundamental understanding of the kinetics and isotopic fractionations of many of the atmospheric sulfur oxidation processes, in which v17 O is becoming a very useful and important parameter. Acknowledgements

[5]

[6]

[7] [8]

[9] [10] [11] [12]

We thank the Ministry of Environment and Tourism of Namibia for giving permits to work in the Namib-Naukluft Park and Rolf Walter for providing some of the samples. This project is supported by NSF and NASA, USA and DFG, Germany (Deutsche Forschungsgemeinschaft). [EB]

[13] [14] [15]

[16]

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